1. Field of the Invention
The present invention relates to an angular position indicating signal generator for a rotary element of a data reproduction machine and, more particularly, to an angular position indicating signal generator for a rotary head apparatus of a video tape recorder (hereafter referred as VTR).
2. Description of the Prior Art
Generally, recording operations and playback operations of video signals in VTRs are carried out by a helical scanning system for a magnetic tape. In this helical scanning system, two rotary heads are rotatively provided in a rotary head apparatus, arranged 180.degree. apart relative to the rotating axis of the rotary heads. The rotary head apparatus has a cylinder in which the rotary heads are coaxially supported and exposed at the outer surface of the cylinder. Thus, each rotary head scans the magnetic tape wound diagonally around the cylinder for an angle of 180.degree.. The rotation speed of the rotary heads is controlled so that each rotary head scans one frame of the video signal.
In this way, in a helical scanning VTR system, the recording operations and playback operations of the video signal are shared by two rotary heads. Thus, for example, it is necessary to detect the angular positions of the rotary heads on a rotating plane of the rotary disc when recording or reproducing continuous signals which combine the video signals of each rotary head on predetermined locations of the magnetic tapes.
Referring now to FIG. 1, there is shown a prior art circuit for generating a signal indicative of the angular position of a rotary head relative to a rotating plane of the rotary head. In FIG. 1, a pair of first and second rotary heads 11a and 11b are provided on outer ends of a rotary disc 12 of a rotary head apparatus. The first and second rotary heads 11a and 11b are separated from each other by an angle of 180.degree. relative to the rotating axis of the rotary disc 12. Also a pair of first and second position referencing magnets 13a and 13b are mounted on the rotary disc 12 corresponding to the first and second rotary heads 11a and 11b, respectively. The first and second position referencing magnets 13a and 13b are located on a circumference with the center of the rotating axis and precede the corresponding rotary heads 11a and 11b by angles .theta.a and .theta.b in the direction of rotation. The first and second position referencing magnets 13a and 13b are directed so that they generate magnetic fields of opposite polarity in the direction of the rotating axis. A magnetic field detection coil 14 is provided facing the circumference so that the first and second rotary heads 11a and 11b alternately face the magnetic field detection coil 14 during the rotation of the rotary disc 12. Thus, the magnetic field detection coil 14 generates a magnetic field detection signal S1 in response to the first and second position referencing magnets 13a and 13b. The magnetic field detection signal S1 includes first and second sinusoidal waveform pulses of opposite polarity, which alternately rise corresponding to the first and second position referencing magnets 13a and 13b, as shown in FIG. 2.
The magnetic field detection coil 14 is connected to a rectangular waveform shaping circuit, e.g., a Schmitt circuit 15. Thus, the detection signal S1 is applied to the Schmitt circuit 15 so that the detection signal S1 is converted to a rectangular waveform signal S2. The rectangular waveform signal S2 falls and rises according to the first and second sinusoidal waveform pulses in the detection signal S1, as shown in FIG. 2. The rectangular waveform signal S2 is applied in parallel to a pair of first and second adjustable delay circuits 16a and 16b directly, and indirectly through an inverter 17. The first and second adjustable delay circuits 16a and 16b are provided for decreasing the influence of discrepancies in the angles .theta.a and .theta.b from their standard angles, as described later.
The first and second adjustable delay circuits 16a and 16b are connected at their control terminals to first and second delay time adjusting circuits 18a and 18b, respectively. Each of the first and second delay time adjusting circuits 18a and 18b includes a variable resistor and a capacitor. Thus, the first and second adjustable delay circuits 16a and 16b generate first and second rectangular pulse signals S3 and S4, respectively. The first rectangular pulse signal S3 includes pulses with a first predetermined pulse width d1 corresponding to the rise of the rectangular waveform signal S2, as shown in FIG. 2. The second rectangular pulse signal S4 includes pulses with a second predetermined pulse width d2 corresponding to the fall of the rectangular waveform signal S2, as shown in FIG. 2.
Further, the first and second delay circuits 16a and 16b are connected at their output terminals to a set terminal S and a reset terminal R of a reset set flip-flop circuit (hereinafter referred as RS flip-flop circuit) 19, respectively. Thus, the RS flip-flop 19 generates a second rectangular waveform signal S5. The second rectangular waveform signal S5 falls and rises in correspondence with the trailing edges of the pulses in the first and second rectangular pulse signals S3 and S4, as shown in FIG. 2. The RS flip-flop circuit 19 is triggered by the first and second rectangular pulse signals S3 and S4 of the first and second adjustable delay circuits 16a and 16b. Thus, finally, an output signal H-SW which indicates the angular positions of the rotary heads 11a and 11b is obtained as an output of the RS flip-flop circuit 19. The angular position indicating signal, i.e., the output signal H-SW, is mainly used as a signal for switching outputs of the rotary heads 11a and 11b and for performing the recording or reproducing operations of video signals of the rotary heads 11a and 11b on predetermined locations of magnetic tapes.
The above described discrepancies of the angles .theta.a and .theta.b often arise in mass-produced VTR structures. In particular, these occur when the mounting accuracy of rotary heads 11a and 11b to the rotary disc 12 is inaccurate. Therefore, the first and second adjustable delay circuits 16a and 16b are installed for decreasing the discrepancies of the angles .theta.a and .theta.b by adjusting the pulse widths d1 and d2 in the first and second rectangular pulse signals S3 and S4.
Incidentally, of these first and second adjustable delay circuits 16a and 16b, the former is for use with first position referencing magnet 13a while the latter is for use with second position referencing magnet 13b.
The adjustment is carried out so that, at a specified time after the head switching signal, the vertical synchronous signal V of the video signal is recorded or reproduced. For example, in the VHS system, the vertical synchronous signal V is provided to be recorded or reproduced after the head switching phase with a delay time specified to a value between 5H-8H (H is the horizontal scanning period or one field period), and generally the delay time is adjusted at the center of the range, i.e., 6.5H. In fact, during the adjustment process in VTR manufacturing, a test tape (i.e., a reference tape) on which the vertical synchronous signal Vr is recorded at a specified position is reproduced, and the delay time is adjusted to the specified value while observing the time difference between the reproduced vertical synchronous signal Vr and the angular position indicating signal H-SW.
However, a variable resistor and a capacitor are required for adjustment in the above prior art angular position indicating signal generator for a rotary head apparatus of the VTR and, in addition, a considerably long time is required for the adjustment. For this reason, it has the drawback that the overall cost is high. Also, even if the adjustment of the variable resistor is automated by using a robot, there are difficulties from the aspect of adjustment accuracy. Moreover, randomness of adjustment occurs due to variations over time during use and variations in temperature characteristics.
Although variations or discrepancies of the angular position indicating signal H-SW due to variations or discrepancies in the angular differences of the angles .theta.a and .theta.b are compensated for in the prior art, as described above, installation parts for adjustment are required, it is difficult to automate the adjustment, and adjustment accuracy is affected by use over time, etc.